The present invention relates to the storage and processing of digital data, and more particularly to a multiple serial access memory for providing a plurality of different data streams from data stored therein. The invention is especially useful in providing a frame store for a motion compensated television system.
Digital transmission of television signals can deliver video and audio services of much higher quality than analog techniques. Digital transmission schemes are particularly advantageous for signals that are broadcast over cable television networks or by satellite to cable television affiliates and/or directly to home satellite television receivers. It is expected that digital television transmitter and receiver systems will replace existing analog systems just as digital compact discs have largely replaced analog phonograph records in the audio industry.
A substantial amount of digital data must be transmitted in any digital television system. This is particularly true where high definition television ("HDTV") is provided. In a digital television system, a subscriber receives the digital data stream via a receiver/descrambler that provides video, audio, and data to the subscriber. In order to most efficiently use the available radio frequency spectrum, it is advantageous to compress the digital television signals to minimize the amount of data that must be transmitted.
The video portion of a television signal comprises a sequence of video images (typically "frames") that together provide a moving picture. In digital television systems, each line of a video frame is defined by a sequence of digital data bits referred to as "pixels." A large amount of data is required to define each video frame of a television signal. For example, 7.4 megabits of data is required to provide one video frame at NTSC (National Television System Committee) resolution. This assumes a 640 pixel by 480 line display is used with 8 bits of intensity value for each of the primary colors red, green, and blue. High definition television requires substantially more data to provide each video frame. In order to manage this amount of data, particularly for HDTV applications, the data must be compressed.
Video compression techniques enable the efficient transmission of digital video signals over conventional communication channels. Such techniques use compression algorithms that take advantage of the correlation among adjacent pixels in order to derive a more efficient representation of the important information in a video signal. The most powerful compression systems not only take advantage of spatial correlation, but can also utilize similarities among adjacent frames to further compact the data.
Motion compensation is one of the most effective tools for accounting for and reducing the amount of temporal redundancy in sequential video frames. One of the most effective ways to apply motion compensation in video compression applications is by differential encoding. In this case, the differences between two consecutive images (e.g., "frames") are attributed to simple movements. The encoder estimates or quantifies these movements by observing the two frames and sends the results to a decoder. The decoder uses the received information to transform the first frame, which is known, in such a way that it can be used to effectively predict the appearance of the second frame, which is unknown.
The encoder reproduces the same prediction frame as the decoder, and then sends the differences between the prediction frame and the actual frame. In this way, the amount of information needed to represent the image sequence can be significantly reduced, particularly when the motion estimation model closely resembles the frame to frame changes that actually occur. This technique can result in a significant reduction in the amount of data that needs to be transmitted once simple coding algorithms are applied to the prediction error signal. An example of such a motion compensated video compression system is described by Ericsson in "Fixed and Adaptive Predictors for Hybrid Predictive/Transform Coding," IEEE Transactions on Communications, Vol. COM-33, No. 12, December 1985. An improvement to the system described by Ericsson is disclosed in commonly assigned U.S. Pat. No. 5,057,916 entitled "Method and Apparatus for Refreshing Motion Compensated Sequential Video Images."
Motion compensated television systems require the storage of both current frame and previous frame video data to implement the differential encoding technique described above. Specifically, prior frame data must be summed with a transmitted difference signal to recreate a full current frame for display. The full current frame must be stored for subsequent use in recreating the next frame from a received difference signal. In the past, a dual memory bank system has been used to implement such systems. The same data is stored in both memory banks, allowing it to be accessed independently from each bank. Such an implementation, however, is costly, particularly in HDTV systems where large amounts of memory are required.
It would be advantageous to provide a memory scheme that enables the implementation of a feedback system without the need for redundant memory banks. It would be further advantageous to provide television apparatus that utilizes such a memory scheme to implement motion compensation in an efficient and economical manner. It would be still further advantageous to provide a method for storing video data in a single memory array for use in connection with a television system using motion compensation.
The present invention provides a multiple serial access dynamic random access memory (RAM) and a television system and method having the aforementioned advantages.